1. Field of the Invention
The present invention relates to a thermal head used for a recording apparatus of a thermal recording method.
2. Description of the Related Art
Generally, a thermal head of this type is arranged such that a plurality of heating resistors each having two lead electrodes at opposite ends thereof are arranged in a horizontal scanning direction for recording, and are laminated on an insulating substrate by means of thin film technology.
The thermal head having the above-described arrangement, which is adopted in various types of thermal printers of the thermal recording method, can be heated by selectively applying a driving current corresponding to an image signal to the plurality of heating resistors via the opposite lead electrodes thereof.
For instance, in a thermal recording apparatus, as thermal recording paper is brought into direct contact with heated areas of the heating resistors to cause coloration, it is possible to obtain a recorded image corresponding to the image signal.
In addition, in a thermal transfer recording apparatus, it is possible to cope with the recording of an image using plain paper by causing the ink of a heat transfer ribbon to be melted by the heated areas of the heating resistors and transferring the image onto the recording paper.
Furthermore, in a recording apparatus using a heat subliming ribbon as a recording medium, a recorded image can be formed by causing the ink of the heat-subliming ribbon to be sublimed by the heating energy and transferred onto the recording paper.
With this type of thermal printer, not only a binary image of black and white but also a half-tone image such as a photograph can be recorded depending on the arrangement of the heating resistors of the thermal head.
FIG. 6 illustrates an example of the structure of a conventional thermal head which is capable of modulating the area of heating dots used in the aforementioned half-tone image recording.
This thermal head is formed by lamination by means of the aforementioned thin film technology. A plurality of heating resistors 2a, 2b, 2c, . . . are arranged on an insulating substrate 1 formed of, for example, a ceramic or alumina in a horizontal scanning direction, and first lead electrodes 3a, 3b, 3c, . . . and second lead electrodes 4a, 4b, 4c, . . . each having a rectangular configuration are respectively connected in series with opposite ends of the heating resistors 2a, 2b, 2c, . . .
The configuration of each of the heating resistors 2a, 2b, 2c, . . . forms a parallelogram which is surrounded by two lines parallel with the direction of their arrangement (horizontal scanning direction) and two mutually parallel lines that intersect those two lines at a predetermined angle.
This configuration has been determined on the basis of the results of various experiments so that a heating distribution characteristic particularly suitable for the heating-dot modulation for half-tone recording can be obtained. During heating and driving, the heating resistor exhibits a current distribution such as the one shown in FIG. 7.
That is, in FIG. 7, points of measurement are indicated by dots, and the direction of a line extending from each dot indicates the direction of the current at that point of measurement, while the length of the line indicates the magnitude of the current at that point of measurement.
It can be appreciated from this diagram that the distribution of a current flowing across a heating resistor 2n, when heated and driven, is denser toward its central portion, and coarser toward its peripheral portion.
This will be apparent from a demonstration taking note of a formula shown below.
Generally, assuming that voltage is v and electrical conductivity is .sigma., current i flowing across a point in the heating resistor 2n, when heated and driven, is expressed by the formula: ##EQU1##
Hence, the voltage v is expressed by the following Laplace equation: ##EQU2##
This Laplace equation can be solved by a boundary element method which is one calculating method using a computer, and a current vector can be determined.
FIG. 7 illustrates a distribution of current vectors determined by the above-described method, and it is apparent that the current becomes larger toward the central portion of the heating resistor 2n, i.e., that the current concentrates.
Here, if its resistance is assumed to be R, a calorific value E at a point in the heating resistor 2n can be expressed by the following formula: EQU E=Ri.sup.2 ( 3)
In other words, the calorific value E is proportional to the square of the current i.
Consequently, in the heating resistor 2n having a current distribution such as the one shown in FIG. 7, the central portion where the current distribution is dense and the current value is large exhibits a conspicuously large calorific value as compared with the peripheral portion where the current distribution is coarse and the current value is small.
Here, the calorific value E changes in correspondence with the value of the current i on the basis of the aforementioned Formula (3), and this calorific value E causes the density of a recording dot during actual recording to change in correspondence with the size of the heating dot at that time.
By making use of this action, it is possible to realize recording with a desired gradation by adjusting the current i to be supplied to the heating resistor 2n in accordance with the density of an image signal.
To effect recording positively with a clear gradation, it is desirable that the response characteristics of the calorific value E with respect to the driving current i be good, and for this purpose it is necessary to maintain the thermal efficiency at as high a level as possible.
In this respect, with the above-described conventional thermal head, the heating resistor 2n does not exhibit a sufficiently high thermal efficiency owing to the factors which will be described below.
That is, in the conventional thermal head, the area of the peripheral portion is large in view of the heating characteristic inside the heating resistor 2n in correspondence with the current distribution shown in FIG. 7, i.e., the characteristic that the calorific value is greater toward the central portion. Hence, thermal diffusion from this peripheral portion is liable to occur.
This type of thermal diffusion occurs due to the fact that the calorific value at the peripheral portion is smaller than at the central portion, and is particularly noticeable in acute-angled portions of the parallelogram where the current distribution is remarkably coarse.
In addition, concerning the same configuration, the first lead electrode 3n and the second lead electrode 4n are formed with a width which is identical with that of that side of the parallelogrammatic heating resistor 2n which extends in the horizontal scanning direction. Hence, the abutting width of each of the lead electrodes 3n, 4n for connection to the heating resistor 2n is very large.
That side of the heating resistor 2n which extends in the horizontal scanning direction includes the aforementioned acute-angled portion of the parallelogram, and tends to display a locally small calorific value.
The fact that the lead electrodes 3n, 4n abut on each side of the heating resistor 2n in this condition with a large width further promotes the aforementioned thermal diffusion.
As a result, the thermal efficiency of the heating resistor 2n has been extremely low, and the heat response characteristic with respect to the driving current has been deteriorated.
Hence, with the recording apparatus using this type of conventional thermal head, there has been a problem in that the resolution decreases in conjunction with the deterioration of the heat response characteristic of the heating resistor 2n, thereby making it impossible to obtain a half-tone recorded image of an excellent quality.